Meeting requests sent but not received

One of my users experiences a strange behaviour with his exchange 2010 account. When he invites another user within the same organisation/same server, the invite is shown as sent but the recipient never actually received it. To make sure it wasn't linked to his outlook profile, we did the same test using his OWA account. Same result. The meeting is created in his calendar, the meeting request is shown as sent.

There's no problem with emails. All emails he send are correctly received.

IS this behaviour with all the users ? (i mean all users in your organization is unable to send invitation ) ?

If its only for single user, then
1> is he sending invitation to user in the same organization ?
2> is he tried sending invitation to other users within organization ?
3> is mails sending to users are fine ?
4> Is this behaviour the first time ?

@Ahmed:
Only one single user is concerned. Same behaviour with same user on different computer and in OWA

1>Yes. Internal invitations
2>No. Will try.
3>mails sending are fine. Totally normal behaviour
4>this is a quite new user. His account has been created a few weeks back. It's difficult to say if it's band new or if he only noticed it recently.

@Mks
The logs show the meeting request has been sent even though the recipient did not receive it
Will check the check box. That would be a first if it happened to be the reason

1> IF the user is unable to send meeting request to only one user then 100% the problem is with recipient, so in this case user may get NDR or may not.

2> IF the user is unable to send meeting request to other recipeints aswell, then most probabely if this is new user then you can try to recreate his profile and check and also you can try below command on his machine.

1. Close Outlook.
2. Go to Start, Run and enter: outlook.exe /cleanrules
3. That starts Outlook
4. Close Outlook again
5. Go to Start, Run and enter: outlook.exe /cleanserverrules

3> You can also try same command on recipeint machine if he is unable to take meeting request from any of the users in your organization.

4> For testing you can send a test meeting from user to itself and check he himself receieves or not.

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